Commit 0fb54de9 thought that this was only needed in VS2015 and later,
but buildfarm member woodlouse shows that at least VS2013 whines as
well.  Let's just define it regardless of MSVC version; it should be
harmless enough in older releases.

Also, in the wake of ed9b3606, it seems better to put it in win32_port.h
where <winsock2.h> is included.

Since this is only suppressing a pedantic compiler warning, I don't
feel a need for a back-patch.

Discussion: https://postgr.es/m/20124.1510850225@sss.pgh.pa.us